[Compatibility] Make Subnet Entity OpenStack Compatible

[kevin-zhonghao]

Issue Decription
Openstack subnet show got error: 'NoneType' object is not iterable.
Using Openstack horizon to show subnet also got
Danger: An error occurred. Please try again later.

Solution

1. Modify AllocationPool.class:
   @JsonProperty("start_ip") -> @JsonProperty("start")
   @JsonProperty("end_ip") -> @JsonProperty("end")

2. Add configuration of default value in creation of subnet.

[codecov-io]

Codecov Report

Merging #438 into master will increase coverage by 0.10%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master     #438      +/-   ##
============================================
+ Coverage     36.78%   36.88%   +0.10%     
- Complexity     1149     1153       +4     
============================================
  Files           447      447              
  Lines         10706    10722      +16     
  Branches       1367     1371       +4     
============================================
+ Hits           3938     3955      +17     
+ Misses         6233     6231       -2     
- Partials        535      536       +1     

Impacted Files	Coverage Δ	Complexity Δ
...rewei/alcor/subnet/utils/SubnetManagementUtil.java	21.80% <75.00%> (+7.27%)	5.00 <0.00> (ø)
...rewei/alcor/portmanager/processor/PortContext.java	69.09% <0.00%> (+1.81%)	23.00% <0.00%> (+1.00%)
...alcor/portmanager/util/RestParameterValidator.java	44.61% <0.00%> (+3.07%)	14.00% <0.00%> (+1.00%)
...i/alcor/portmanager/processor/RouterProcessor.java	84.37% <0.00%> (+6.25%)	10.00% <0.00%> (+1.00%)

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[cj-chung]

LGTM